Burning reveals cryptic plant diversity and promotes coexistence in a California prairie restoration experiment

Authors: YOUNG, DEREK - University Of California; Porensky, Lauren; WOLF, KRISTINA - University Of California; FICK, STEPHEN - University Of California; YOUNG, TRUMAN - University Of California

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/29/2014
Publication Date: 5/26/2015
Publication URL: http://handle.nal.usda.gov/10113/60989
Citation: Young, D.J., Porensky, L.M., Wolf, K.M., Fick, S.E., Young, T.P. 2015. Burning reveals cryptic plant diversity and promotes coexistence in a California prairie restoration experiment. Ecosphere. 6(5):81. http//dx.doi.org/10.1890/ES14-00303.1.

Interpretive Summary: Grassland and prairie restoration projects in California often result in long-term establishment of only a few native plant species, even when they begin with a diverse seed palette. In many cases, diversity reductions occur because less competitive (subordinate) species get out-competed by their more competitive (dominant) neighbors. Spatially patchy plantings and fire are two strategies that may boost long-term diversity by helping subordinate species escape dominant species in either space or time. However, no studies have experimentally evaluated the effects of fire on a restored community or the interactions between fire and spatially patchy plantings. In a previous study, we demonstrated that patchy plantings increased community diversity for three years. However, diversity declined throughout the study, and the benefits of patchy plantings had begun to disappear by the third year. In the current study, we resurveyed the experimental plots five years after seeding and in the following year carried out controlled burns on half of the plots. Several subordinate species continued to decline and essentially disappeared aboveground by the fifth year, as did one of the previously dominant species. Controlled burns decreased the cover of dominant species and triggered the reappearance of three subordinate species, which were apparently still present as viable seeds stored in the soil seed bank. As a result of these shifts, fire increased overall community diversity. Spatial planting treatments (uniform or patchy) did not affect community-level responses to fire. These results confirm that although patchy plantings may promote species coexistence in the short term, re-establishing fire and other important disturbances can allow coexistence over a longer time scale by revealing and renewing seed bank diversity.

Technical Abstract: Grassland and prairie restoration projects in California often result in long-term establishment of only a few native plant species, even when they begin with a diverse seed palette. A likely explanation for the disappearance of certain native species over time is that they are excluded through competition. If so, management that reduces interspecific competition may favor “subordinate” natives and promote greater native species diversity in restored communities. Potential management approaches to accomplish this goal include intraspecific spatial aggregation during seeding and prescribed fire. However, no studies have experimentally evaluated the effects of fire on a controlled species pool or the interaction between fire and spatial aggregation. In a previous California prairie restoration experiment, we demonstrated that aggregated plantings protected competitively subordinate species from exclusion and increased community diversity for three years. However, species richness declined throughout the study, and the benefits of aggregated seeding had begun to disappear by the third year. For the present study, we resurveyed the experimental plots five years after seeding and in the following year carried out controlled burns on half of the plots. Of the three subordinate species that had become rarer each year in the first three years of the study, all continued to decline and essentially disappeared aboveground over the following two years, as did one of the previously dominant species. However, burning triggered the reappearance of the three subordinate species that had disappeared or nearly disappeared in previous years, decreased the cover of dominant natives, and as a result increased community diversity. However, seeding treatments (aggregated or interspersed) did not affect community-level responses to the burning treatment. These results confirm that although initial intraspecific aggregation may promote species coexistence in the short term, re-establishing disturbance regimes can allow coexistence over a longer time scale by revealing and renewing seed bank diversity.